Nondestructive evaluation of materials and bonds frequently requires the transmission of ultrasound waves into the material or bond to be evaluated. Ultrasound waves may be transmitted into the material by placing a transducer, which generates ultrasound waves, on a surface of the article under test. One form of ultrasound testing involves transmitting ultrasound shear waves into the material under test, with the wave propagation direction perpendicular to the surface of the material under test. The shear waves generated by a shear wave transducer may provide greater spatial resolution than the more commonly used longitudinal waves at a given frequency, and can provide other complementary data.
Acoustic impedance mismatch between ambient air and the solids, such as the material of the article under test, may be large. In such circumstances, nearly all of the energy generated by the transducer in the form of ultrasound shear waves may be reflected and very little transmitted into the article under test. In order to counteract this effect, a couplant is employed to displace the air between the transducer and the surface of the article under test, thereby making it possible to transmit more sound energy into the test specimen so that a useable ultrasonic signal can be obtained.
In contact ultrasonic testing where the face of a transducer body through which sound waves are transmitted is placed against a corresponding surface of the material under test, the couplant may take the form of a thin film of oil, glycerin or water that is placed between the face of the transducer body and the surface of the material under test. Other types of couplants may include the use of a rigid adhesive such as an epoxy, or a highly viscous “honey” couplant.
Such couplants may possess disadvantages. For example, use of an epoxy couplant may result in damage either to the face of the transducer or the article under test upon removal of the transducer from the article under test. Further, use of couplants that are highly viscous, or use of couplants such as glycerin, oil, or water, may be messy and may contaminate the surface of the article under test.
The lack of a feasible couplant for use with ultrasound shear wave transducers has discouraged the use of ultrasound shear wave transducers in spite of the benefits of using ultrasound shear waves to evaluate an article under test. Accordingly, there is a need for a suitable couplant for use with an ultrasound shear wave transducer, as well as other types of transducers.